Electroradiographic process

ABSTRACT

A process by which the radiographic radiation applied to an object for exposure thereof is used to form directly and indirectly a conductivity pattern on a conductive grid comprising a wire mesh completely covered with a photoconductive insulating material. A flow of ions directed toward the grid is modulated to produce a latent electrostatic image on the surface of an image receiving material, the latter and a radiographic fluorescent material are arranged in close proximity to said grid and in contact with a conductive backing member.

United States Patent 2,900,515 8/1959 Criscuoloetal 3,220,833 11/1965 McFarlane Primary Examiner-WilliamF. Lindquist AttorneysWilliam H. J. Kline, Robert F. Crocker and Lloyd F Seebach ABSTRACT: A process by which the radiographic radiation applied to an object for exposure thereof is used to form directly and indirectly a conductivity pattern on a conductive grid comprising a wire mesh cor'npletely covered with a photoconductive insulating material. A flow of ions directed toward the grid is modulated to produce a latent electrostatic image on the surface of an image receiving material, the latter and a radiographic fluorescent material are arranged in close proximity to said grid and in contact with a conductive backing member.

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52 00 9 Electrode HENRY M CLEARE INVENTOR.

ELECTRORADIOGRAPHIC PROCESS 1. Field of the Invention The present invention relates toradiography and more particularly to an electroradiographic process that is more sensitive to radiographic exposure. 2. Description of the Prior Art Electroracliography is similar to electrophotography in that the sequence of operations and the materials used are substantially the same. The techniques that are used are also similar in many respects. In most electroradiographic processes a conventional xerographic plate is used which comprises a conductive member having a photoconductive insulating layer coated on one surface thereof. The photoconductivelayer is first sen 'sitized by corona charging in a well-known manner. An object to be X-rayed is then positioned in proper spatial relation between the photoconductive layer on the plate and a source of X-rays. Upon exposure to X-rays some of theX-rays pass through the object and penetrate into the photoconductive layer on the plate. The X-rays that pass through the object produce a latent electrostatic image on the plate which can then be made visible by developing the image with a contrasting and suitable powder. The developed image can be one that is viewed directly on the plate, or it can be one that is transferred to a sheet of paper'to provide a permanent record. In the latter case, the electroradiographic plate can be reused if, after transfer of the image, all of the residual powder is removed and the photoconductive surface is thoroughly cleaned.

Another electroradiographic process is one that is based on the ionization of air caused by X-rays and does not require a photosensitive layer. In this process, the X-ray image is recorded on a sheet of insulating material, such as vinyl plastic, that is placed in contact with a conductive member or has a conductive material coated on one surface of the insulating material. The X-ray sensitivity is achieved by placing an electrostatic charge on the surface of the insulating sheet and then placing it relative to one side of a wire mesh screen, the objectto be X-rayed being arranged on the other side of the screen and between the screenand a source of X-rays. The X- rays that pass into the parallel-plate chamber formed by the screen and plate, ionize the air in this space due to the electrical field that exists in the chamber. If the plate surface has a positive charge, negative ions-will be drawn to the plate where they are neutralized by the positive charge, or vice versa. The ionization that takes place is dependent on the intensity of the X-rays emerging from or passing through the object. Thus, the discharge of the plate will be greater in areas corresponding to voids or cavities in the object. The resulting latent electrostatic image on the plate can then be developed as described above.

With either of the aforementioned systems, a charge must be placed on a surface of the sheet of insulating material before the exposure step can be accomplished. Further, a relatively long exposure time is required to obtain an image of at least fair density, because the sensitivity of a photoconductive plate to X-rays'is considerably less than, and hence, slower in speed than that of a photographic X-ray film plate. The use of lead or fluorescent screens is considered to be a practical procedure in ordinary film radiography. However, in electroradiography when such a screen is placed in contact with a sensitized or charged plate, the surface charge is dissipated to the extent that the plate is not usable. Also, X-ray exposure produces a temporary effect in a photoconductive layer known as fatigue which can be eliminated by exposing the layer briefly to the heat from an infrared lamp. However, this necessitates the addition of still another step to the process.

SUMMARY OF THE INVENTION The primary object of the present inventionis to provide a process by which a radiographic reproduction of an object can be made with 'a considerable decrease in exposure time.

Another object of the invention is to provide a process by which'the'effect of radiographic exposure on an electrically insulating receiving sheet can be utilized indirectly to increase the intensity of exposure of a photoconductive material.

Still another object of the invention is to provide a process by which a layer of material arranged relative to a photoconductive grid increases the intensity of exposure of the grid indirectly over and above that of the radiographic energy incident directly on the grid. 7

These and other objects and advantages of the invention will be apparent to those skilled in the art by the more detailed description which follows hereinafter.

The objects of the invention are attained by utilizing an image receiver which comprises a phosphor material dispersed in an insulating binder layer and coated on a sheet of conductive material, e.g., conductive paper, or a sheet of conductive paper having a coating on one surface comprising a conductive binder layer in which a phosphor material has been dispersed, the binder layer having a coating of insulating material thereover or a removable sheet of insulating material in contact therewith. The image receiver or image receiving sheet is placed on an electrode and in close proximity to a photoconductive grid comprising a normally conductive wire mesh which is completely covered with a photoconductive insulating material. The object to be X-rayed is arranged between the grid and a source of radiographic radiation. During exposure of the object to the radiographic radiation, the rays passing through the object render corresponding areas of the grid conductive by direct action and on passing through the grid to the insulating layer excite the phosphor material to provide an indirect exposure of the grid from the opposite side. As a result, a conductivity pattern on the grid is formed not only by direct exposure to the rays, but also by an indirect exposure caused by the radiant energy emitted from the phosphor material due to the excitation thereof by the rays. Sufficient radiant energy is produced by the phosphor material to increase the conductivity of the pattern on the grid, thereby effecting an increase in sensitivity by utilizing the radiographic radiation more effectively. A flow of ions is directed toward the photoconductive grid and the insulating layer to produce a latent electrostatic image on the surface of the image receiving sheet. As described in more detail hereinafter, the conductivity of the areas of the grid that are exposed to the rays modulates the flow of ions so that the resulting latent electrostatic charge pattern on the image receiving sheet is in accordance with the nonconductive areas on the grid. Exposure of the grid and the flow of ions directly toward the grid and image receiving sheet can occur simultaneously or exposure can precede the charging step, provided the photoconductive material on the grid has persistence of photoconductivity.

DESCRIPTION UP THE DRAWING Reference is now made to the accompanying drawings wherein like numerals designate like parts and wherein:

FIG. 1 is a schematic representation of an electroradiographic system showing an insulating material with a phosphor material dispersed therein positioned relative to a photoconductive grid for radiographic exposure with an alternative arrangement of the object being X-rayed and the source of X- radiation relative to the photoconductive grid being shown in dotted lines;

FIG. 2 is an enlarged section through an image receiver comprising a transparent insulating sheet that is separable from a conducting material having a conductive coating on one surface thereof in which a phosphor material is dispersed or incorporated; and

FIG. 3 is a schematic representation similar to that shown in FIG. 1 of an electroradiographic system in which an additional screen has been interposed between the corona means and the photoconductive grid.

DESCRIPTION OF THE INVENTION With particular reference to FIG. 1, a grid 8 is arranged between a backing electrode 22 and at least one corona discharge electrode 16. While the entrance electrode 16 is shown as being generally central relative to the overall system, the electrode can be arranged at one side to prevent any shadow effect from the radiographic radiation. An object 24 that is to be X-radiation is positioned between the corona discharge electrode 16 and a source of X-radiation 26. The photoconductive grid 8 comprises a conductive wire mesh core 10 that is completely coated with a layer of photoconductive insulating material 12 which is sensitive to the radiant energies used in the process. The photoconductive coating 12 must have no holes or cracks therein and must completely cover the core 10. A source of potential 14 has one terminal connected to corona discharge electrode 16 and the other terminal connected to ground 18 through a bias voltage source 20. The core 10 is connected through voltage source 20 to ground 18 and the backing electrode 22 is also connected to ground 18.

An image receiving sheet 2 comprises a conductive paper support 4 on one surface of which is coated a layer 6 of insulating material having fluorescent material 6 incorporated therein. The image receiving sheet 2 is positioned on backing electrode 22 with the layer 6 adjacent the grid 8. The surface of sheet 2 adjacent grid 8 should be as close as possible to the grid in order to eliminate the possibility of image spread due to the spatial separation of the image receiving sheet 2 and grid 8, thereby reducing the image resolution as will be more evident from the description which follows. The insulating material 6 must be capable of holding a charge for a length of time that is sufficient to permit development of the latent electrostatic image placed thereon by the X-rays.

When the object 24 is exposed to X-rays a conductive image pattern is formed on grid 8 which serves to modulate the flow of ions to the sheet 2. The X-rays which pass through the object and are incident on the image receiving sheet 2, excite the phosphor material 6' therein which, in turn, emits radiant energy toward the photoconductive grid 8, thereby making it become more conductive in the imagewise exposed portions. Accordingly, higher conductivity is achieved in the imagewise exposed portions of the grid 8 as well as greater control of the imagewise charge pattern. With the same amount of X-radiation exposure, the grid 8 attains a much higher level of conductivity with the use of a phosphor material than without. As a result, more charge can reach the image receiving sheet so that an increase in exposure speed is obtained with the use of the phosphor material.

The corona discharge electrode 16 is energized to produce a flow of ions either while the grid 8 is being exposed to X-rays or, if the photoconductive material 12 is one that exhibits persistence of photoconductivity, at any time while the photoconductive pattern or image exists on the grid 8. The imagewise exposed photoconductive grid 8 modulates the flow of ions from corona discharge electrode 16 to produce a latent electrostatic image on the surface of image receiving sheet 2. This modulation of the ion flow is due to the fact that the portions of the photoconductive grid 8 on which no X-rays are incident remain as insulating portions and the insulating portions that have X-rays incident thereon become conducting. The ions that collect at the conducting portions of the grid essentially leak off to ground. On the other hand, the ions that strike the insulating portions of grid 18 accumulate and build up a surface potential on those portions of grid 8. When the potential buildup is generally greater than the bias voltage 20, the ions can then pass through grid 8 in the insulating portions to the surface of the image receiving sheet 2 immediately therebelow to form a latent electrostatic image.

As shown in dotted lines in FIG. '1, an object 24 and a source of X-radiation 26can be arranged on the other side of the backing electrode 22 with substantially the same results being obtained. With this arrangement, however, a greater proportion of the exposure is due to the radiant energy from the phosphor material because the phosphor material is highly absorbing for the X-radiation. However, the X-ray absorption of the grid and the corona does not interfere with the exposure pattern.

As shown in FIG. 2, a removable, transparent image receiving sheet 30 is positioned in overlying contact with a conductive binder layer 32 having a phosphor material 34 incorporated or dispersed therein, the layer 32 being coated on a conductive paper backing 36 which, in turn, is positioned in overlying contact with a backing electrode 38. Since the sheet 30 is removable, the layer 32 is reusable and can be a permanent part of the apparatus, e.g., it can be coated on the backing electrode 38. Also, the receiving sheet 30 must be transparent because the radiant energy emitted by the excited phosphor material 34 in layer 32 must pass through receiving sheet 30 to the photoconductor grid. This arrangement of a transparent image receiving sheet that is used in conjunction with a sheet carrying a phosphor material and forming a party of the system with a backing electrode can be substituted for the image receiving sheet 2 and electrode 22 shown in FIG. 1.

With reference to FIG. 3, the image receiving sheet 42 comprises an insulating material 46 with a phosphor material 46' incorporated therein and coated on a conductive paper backing or support 44, as shown in FIG. 1. The receiving sheet 42 is arranged on a backing electrode 62 and positioned in close proximity to a photoconductive grid 48 comprising a core 50 that is coated with a photoconductive insulating material 52. The backing electrode 62 is connected to ground 58 and a corona discharge electrode 56 is connected to a 10 kv. source of positive potential when the grid 48 is being charged, to ground during X-radiographic exposure of the grid 48, and to a 10 kv. source of negative potential when the conductivity pattern on grid 48 is being used to modulate the flow of ions to the image receiving sheet 42. The object 64 that is to be X-rayed is arranged between corona electrode 56 and a source of X-radiation 66, the electrode 56 being arranged in a position relative to receiving sheet 42 so as to produce a minimum shadow effect upon the grid 48. In this embodiment an electrically conductive screen 68 is interposed between the grid 48 and corona discharge electrode 56.

While the exposure of the receiving sheet 42 is described in some detail hereinbelow, a more complete description and disclosure of an electrographic system somewhat similar to that disclosed in FIG. 3 can be found in a copending application, Ser. No. 492,988, filed Sept. 27, 1965, by L. F. Frank. In the arrangement disclosed herein, the screen 68 and the backing electrode 62 are at ground potential during charging of the grid 48 and during X-radiographic exposure of the grid. The core 50 of the grid is connected to a 300 volts while the electrode 56 is connected to a positive 10 kv. source. After the grid 48 has been charged, the object 64 is exposed to the source of X-radiation 66 and this exposure results in producing an imagewise conductivity pattern on the grid 48, the X- rays passing through the object also passing through the grid 48 to the phosphor material 46' which is then excited, so that the conductive areas of the grid are rendered more conductive by the radiant energy emitted by the phosphor material. In order to place an imagewise latent electrostatic charge on the receiver sheet 42, the corona discharge electrode 56 is energized with a minus 10 kv., a l500 volts being connected to the screen 68, a -l750 volts being connected to the grid 48 and the electrode 62 being connected to ground.

As in the arrangement shown in FIG. 1, the grid 48 and the screen 68 modulate the flow of ions to the surface of the insulating material 46 to produce a latent electrostatic image on the surface thereof in accordance with the X-ray image of the object. When the grid 48 is charged, the photoconductive layer 52 is charged to about 300 v. When the photoconductive grid 48 is imagewise exposed, the areas that are exposed to X- rays are reduced to a charge of about 230 volts, whereas the areas that are not exposed to X-rays retain their charge of about 300 v. The X-rays that continue on through the grid 48 and penetrate the insulating material 46 excite the phosphor material 46' so that the photoconductive material comprising the conductivity pattern on the grid 48 is rendered more conductive by the radiant energy emitted by the phosphor. With the energization of the corona discharge electrode 56 and the voltage applied to the screen 68 and the grid 48 as mentioned above, the voltage differential between grid 48 and the screen 68 is about 250 V. In the unexposed areasor insulating portions of the grid 48, the difference in potential between the grid 48 and screen 68 creates an electrical field of about 50v. which is sufficient to divert. the ions to screen 68 thereby preventing their passing through the grid 48. On the other hand, there is a voltage differential of about 20 v. between the exposed areas or conductingiportions of the grid48 and the screen 68, so that in theseareas an electrical field aids the flow of ions through the grid 48 to the surface of the insulating material 46. The resulting latent electrostatic image on the surface of insulating material 46 can then be developed into a visible image by any one of several known xerographic processes. It is to be understood that the voltages enumerated above as being applied to thevarious elements are only illustrative and are not to beconsidered as any operating limitation.

The following examples disclose various materials and exposure parameters by whichimage receiving sheets in accordance with the invention were exposed and developed to produce an electroradiographic image. Although the aforementioned apparatus and process have been described relative to X-radiation as the exposure medium, other types of radiation classified as short wave, electromagnetic radiation, which includes gamma rays, beta rays, etc., can be used. However, such rays must be capable of exciting an energy converting material that will produce radiant energy.

EXAMPLE 1 An insulating receiving sheet containing a phosphor materi-- al was prepared in the following manner: to 50 grams polyester, percent in a solvent mixture of methylenechloride (80 percent) trichloroethylene per cent), was added 50 grams of radiographic fluorescent powder, such as barium lead sulfate BaPbSO.,. This was mixed in a water jacketed blender, the powder-to-binder ratio was 10:1 and the percent solids equalled 55 percent. The mixture was blended for 10 minutes at 20 C., filtered through silk bolting cloth and coated on 0.008-inch paper stock. The coating was air dried in a hood.

The coated sheet was placed in relation to an object and grid as shown in FIG. 1 with a voltage of 7 kv. applied to the electrode 16 and a bias of +10 v. applied to the grid 8. The object was exposed for 0.1 second to an X-radiation source at a distance of 40 inches therefrom, the X-radiation source being energized by 80 kvp. (kilovolt peak) and 100 ma. A second receiving sheet, prepared as above but without the X-ray fluorescent powder, was given the same exposure under the same conditions. The two receiving sheets were then developed with an electrostatic developer for 30 seconds to give positive images which differed considerably in density and contrast. By making a series of prints with varied exposures, it was determined that with the use of a phosphor coating about one-quarter the exposure time was required for making a print of substantially the same density as one made without a phosphor material. Polyvinyl acetate was also used successfully as a binder for the above fluorescent powder.

EXAMPLE 2 An insulating receiving sheet positioned in contact with a conductive binder layer having a phosphor material dispersed therein and coated on a conductive paper substantially as shown in FIG. 2, was placed in the apparatus shown in FIG. 1. Values as set forth in Example 1 above were used with a similar difference in density and contract. After making a series of prints, it was found that when the phosphor layer was used an increase in exposure speed by a factor of about 4 produced printsof substantially the same density as those made with an ordinary xerographic plate.

Further, it was determined that the position of the layer containing the phosphor material, whether under a transparent receiving sheet, integrated with the receiving sheet, or in combination with the receiving sheet was immaterial because the same desired results were produced. Hence, any of these arrangements of the transparent image receiving sheet is deemed to be within the scope of this invention. In other words, the layer of phosphor material can be immediately adjacent (FIG. 1) the photoconductive grid or can be separated therefrom by a sheet of transparent imagereceiving material as shown in FIG. 2. In either case, the layer of phosphor material is considered to be in close proximity to the photocon'ductive grid.

The invention has been described in detail with particular referenceto preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit nd scope of the invention.

lclaim:

1. An electroradiographic process for producing on an image receiving element a latent electrostatic image corresponding to a radiographic image of an object, comprising:

positioning said image receiving element between a photoconductive grid and a conductive backing member and in contact with said conductive backing member;

directing short wave penetrating electromagnetic radiation through said object and toward said grid while holding said image receiving element in close proximity to said grid for exposing said grid to form a conductivity pattern on said grid corresponding to the radiographic image of said object; and

flooding said grid with ions while maintaining a predeterminedelectric field between said grid and said conductive backing member for modulating a flow of ions in accordance with said conductivity pattern through said grid to said image receiving elements for producing thereon a latent electrostatic image corresponding to the radiographic image of said object.

2. The electroradiographic process according to claim I, including the step of xerographically developing said latent electrostatic image on said image receiving element to produce a visual image.

3. The electroradiographic process according to claim 1 wherein'said image receiving element comprisesa support coated on one surface thereof with a layer of an insulating material having a radiographic fluorescent material dispersed therein for emitting secondary radiation upon being excited by said penetrating electromagnetic radiation during said directing step to further expose said grid from the side adjacent said layer of insulating material.

4. The electroradiographic process according to claim 1 wherein said image receiving element comprises a layer of insulating material in contact with a layer of electrically conductive material with a radiographic fluorescent material dispersed therein, including the step of removing said layer of insulating material from said image receiving element after said latent electrostatic image has been formed thereon.

5. The electroradiographic process according to claim 4 wherein said conductivity pattern is formed directly by said short wave penetrating electromagnetic radiation and indirectly by the secondary radiation emitted by said radiographic fluorescent material upon excitation by said short wave penetrating electromagnetic radiation.

6. The electroradiographic process according to claim 1 wherein the steps of directing short wave penetrating electromagnetic radiation through said object and of flooding said grid with ions occur simultaneously.

7. The electroradiographic process according to claim 1 wherein flooding said grid with ions is subsequent to directing said short wave penetrating electromagnetic radiation through said object and during the persistence of photoconductivity of said conductivity pattern on said grid.

8. The electroradiographic process according to claim 1 wherein said object is positioned on the same side of said grid as said image receiving element with the latter therebetween.

9. The electroradiographic process according to claim 1 wherein said object is positioned on one side of said grid and said image receiving element and said backing member are positioned on the other side of said grid.

10. An electroradiographic process for producing on a layer of insulating material, in contact with an electrically conductive member, a latent electrostatic image corresponding to a radiographic image of an object, comprising the steps of:

producing a conductivity pattern corresponding to said radiographic image on a grid completely covered with an insulating photoconductive material and arranged in spaced and generally parallel relation to said layer of insulating material;

positioning a conductive screen in spaced and generally parallel relation to the side of said grid opposite that facing said layer of insulating material;

directing the flow of ions through said screen and said grid toward said layer of insulating material;

applying an electrical potential difference between said grid and said screen of such a value that the flow of ions through said screen and said grid is modulated in accordance with said conductivity pattern to produce a corresponding latent electrostatic image on said layer of insulating material.

11. An electroradiographic process for reproducing a conductivity pattern, which corresponds to a radiographic image of an object, on a grid comprising a conductive core completely covered with a photoconductive insulating material as a latent electrostatic image on a surface of an image receiving element, comprising the steps of:

positioning said image receiving element substantially parallel to and in close proximity to the surface of said grid on the side opposite said object, said image receiving element comprising a conductive support and a layer ofinsulating material having a radiographic fluorescent material admixed therein that is in contact with the surface of said conductive support facing said grid, the other surface of said conductive support being in contact with a conductive backing member;

directing short wave penetrating electromagnetic radiation through said object toward said grid to expose said photoconductive insulating material, while said image receiving element is so positioned, whereby said photoconductive insulating material is exposed directly from one side by an imagewise pattern of said electromagnetic radiation and indirectly from the other side by secondary radiation emitted by said fluorescent material upon excitation by said electromagnetic radiation passing through said grid; and

flooding said grid with ions, while maintaining a predetermined electric field between said grid and said conductive backing ember for modulating a flow of ions in accordance with said conductivity pattern through said grid to said image receiving element, for producing a latent electrostatic image corresponding to the conductivity pattern on said grid on the surface of said image receiving element.

12. The electroradiographic process according to claim 11 including the step of positioning an electrically conductive screen in spaced and generally parallel relation to the side of said grid opposite said image receiving element and between the source of said ions and said grid during said flooding step.

13. The electroradiographic process according to claim 11 including the step of xerographically developing said latent electrostatic image on said image receiving element into a visual image. 

1. An electroradiographic process for producing on an image receiving element a latent electrostatic image corresponding to a radiographic image of an object, comprising: positioning said image receiving element between a photoconductive grid and a conductive backing member and in contact with said conductive backing member; directing short wave penetrating electromagnetic radiation through said object and toward said grid while holding said image receiving element in close proximity to said grid for exposing said grid to form a conductivity pattern on said grid corresponding to the radiographic image of said object; and flooding said grid with ions while maintaining a predetermined electric field between said grid and said conductive backing member for Modulating a flow of ions in accordance with said conductivity pattern through said grid to said image receiving elements for producing thereon a latent electrostatic image corresponding to the radiographic image of said object.
 2. The electroradiographic process according to claim 1, including the step of xerographically developing said latent electrostatic image on said image receiving element to produce a visual image.
 3. The electroradiographic process according to claim 1 wherein said image receiving element comprises a support coated on one surface thereof with a layer of an insulating material having a radiographic fluorescent material dispersed therein for emitting secondary radiation upon being excited by said penetrating electromagnetic radiation during said directing step to further expose said grid from the side adjacent said layer of insulating material.
 4. The electroradiographic process according to claim 1 wherein said image receiving element comprises a layer of insulating material in contact with a layer of electrically conductive material with a radiographic fluorescent material dispersed therein, including the step of removing said layer of insulating material from said image receiving element after said latent electrostatic image has been formed thereon.
 5. The electroradiographic process according to claim 4 wherein said conductivity pattern is formed directly by said short wave penetrating electromagnetic radiation and indirectly by the secondary radiation emitted by said radiographic fluorescent material upon excitation by said short wave penetrating electromagnetic radiation.
 6. The electroradiographic process according to claim 1 wherein the steps of directing short wave penetrating electromagnetic radiation through said object and of flooding said grid with ions occur simultaneously.
 7. The electroradiographic process according to claim 1 wherein flooding said grid with ions is subsequent to directing said short wave penetrating electromagnetic radiation through said object and during the persistence of photoconductivity of said conductivity pattern on said grid.
 8. The electroradiographic process according to claim 1 wherein said object is positioned on the same side of said grid as said image receiving element with the latter therebetween.
 9. The electroradiographic process according to claim 1 wherein said object is positioned on one side of said grid and said image receiving element and said backing member are positioned on the other side of said grid.
 10. An electroradiographic process for producing on a layer of insulating material, in contact with an electrically conductive member, a latent electrostatic image corresponding to a radiographic image of an object, comprising the steps of: producing a conductivity pattern corresponding to said radiographic image on a grid completely covered with an insulating photoconductive material and arranged in spaced and generally parallel relation to said layer of insulating material; positioning a conductive screen in spaced and generally parallel relation to the side of said grid opposite that facing said layer of insulating material; directing the flow of ions through said screen and said grid toward said layer of insulating material; applying an electrical potential difference between said grid and said screen of such a value that the flow of ions through said screen and said grid is modulated in accordance with said conductivity pattern to produce a corresponding latent electrostatic image on said layer of insulating material.
 11. An electroradiographic process for reproducing a conductivity pattern, which corresponds to a radiographic image of an object, on a grid comprising a conductive core completely covered with a photoconductive insulating material as a latent electrostatic image on a surface of an image receiving element, comprising the steps of: positioning said image receiving element substantially parallel to and in close proximity to tHe surface of said grid on the side opposite said object, said image receiving element comprising a conductive support and a layer of insulating material having a radiographic fluorescent material admixed therein that is in contact with the surface of said conductive support facing said grid, the other surface of said conductive support being in contact with a conductive backing member; directing short wave penetrating electromagnetic radiation through said object toward said grid to expose said photoconductive insulating material, while said image receiving element is so positioned, whereby said photoconductive insulating material is exposed directly from one side by an imagewise pattern of said electromagnetic radiation and indirectly from the other side by secondary radiation emitted by said fluorescent material upon excitation by said electromagnetic radiation passing through said grid; and flooding said grid with ions, while maintaining a predetermined electric field between said grid and said conductive backing ember for modulating a flow of ions in accordance with said conductivity pattern through said grid to said image receiving element, for producing a latent electrostatic image corresponding to the conductivity pattern on said grid on the surface of said image receiving element.
 12. The electroradiographic process according to claim 11 including the step of positioning an electrically conductive screen in spaced and generally parallel relation to the side of said grid opposite said image receiving element and between the source of said ions and said grid during said flooding step.
 13. The electroradiographic process according to claim 11 including the step of xerographically developing said latent electrostatic image on said image receiving element into a visual image. 